Frequency meter



Jam 1943. I. WOLFF 2,307,316

FREQUENCY METER Filed Feb. 13, 1941 8/ G/V/IL SOURCE the signal strength of the reflected waves varies Which s shunted y a capacitor C3 to the elec- Patentecl Jan- 5, 1943 v i UNE'E'ED STATES PATENT FREQUENCY METER Irving Wolfi, Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application February 13,1941, Serial No. 378,713 10- Claims. (Cl. 172-245) This invention relates to frequency meters, and other object is to provide frequency counting particularly to a peak meter for counting or means in which a slow decay period maintains a indicating the beat frequency of a signal notsteady indication notwithstanding variation in withstanding momentary fading. the strength f the applied signal. An additional Various types of frequency counters have been 5 object is to rovide means in a radio altimeter used to indicate the frequency of a signal. One or radio drift meter for indicating the altitude such counter is disclosed in U. S. Patent No. or drift substantial independent of inherent vari- 2,228,367, which issued on January 14, 1941, on ations in signal strength. application Serial No. 248,577, filed December 30, The invention will be described by referring 1938, by Royden C. Sanders, Jr., for Improveto the accompanying drawing in which Figure 1 ment in frequency meters. One of the uses of is a circuit diagram representing a preferred ema frequency counter is in frequency modulated bodiment of the invention, and Figure 2 is a altimeters. A frequency modulated altimeter circuit diagram of a modification. Similar refoperates'in the following manner: A radio transerence characters will be applied to similar elemitter on the aircraft is used to radiate a frements of the drawing.

quency modulated wave toward the earth to be Referring toFig. l, comprised of the beat reflected therefrom. The frequency of the frequency produced from the modulation of sigtransmitted wave is varied continuously. Aradio nals from a frequency modulated transmitter receiver located on the aircraft is made re and a reflected wave thereof, a source of signals sponsive directly to the transmitted wave and .i is prefe ab y ne d o a thermionic amp to the reflected wave. Since the" frequency is befier or repeater 3. The output of the amplifier ing varied, it follows that the directly received is co nected h o h a t ans o mer 5 to a s wave will differ in frequency from the reflected pus discharge tube (3 or to any suitable current wave. The frequency difference is small for limiter. The upper electrode '1 of the gas tube low altitudes and for higher altitudes the freis connected through a capacitor CI to th no quency difierence increases up to the maximum of a diode rectifier Di. The cathode of the altitude for which the altimeter is designed. The present rectifier, as well as rectifiers hereinafter frequency difierence results in a beat frequency mentioned, may be biased by a battery 9, which which may be determined. overcomes any contact potential and insures While the altitude may be det r in by substantially perfect unilateral conductivity. measuring the beat frequency in a number of The cathode of diode Di, or bias battery if one ways, it is generally most convenient to count I is used, is connected by the other electrode H the beat frequency by means of a frequency of the gas tube. The connector of the capacitor counter, as, for example, the'device of the above Cl and anode of the diode DI is joined to the entitled Sanders patent. In practice, some difii- 3 cathode of a second diode D2. The anode of the culties are experienced; it has been found that second diode is connected through a, resistor RI periodically at an irregular rate. The rate of trode ll of the gas tube. The anode of the secvariation appears to depend upon the nature of 0nd diode is connected to the cathode of a third the earth reflecting surface and the altitude. 40 iode D3- The anode of the third diode is con- This variation of signal strength may take place nect d th o a pa tor C to the electrode in a fraction of a second to several seconds. The H. e anode of 3 i a so n ect d h u signals during these periods may fade out coma resistor R2 and meter l3 to the electrode ll pletely or partially. In any event, complete fadof the gas tube. ing is always sufficient to cause a loss of counts The circuit of Fig. 2 is similar to that of Fig. 1 and a loss of counts indicates an apparent loss with the following exceptions: A limiter a p of altitude. Partial fading may cause a, similar fler I5 has been substituted for the amplifier 3 loss. and gas tube G. Rectifiers Kl, K2 and K3 have It is, therefore, an object of this invention to been substituted for the diodes Dl, D2 and D3, provide means for indicating beat frequency correspectively. Rectifiers with substantially perrectly even though the signal is not present part feet unilateral conductivity are preferred, in of the time. Another object is to provide means which case the battery 9 may be omitted. The for indicating the rate of current pulses over 9. operation to be described by referring to Fig. -1 substantial time interval and to prevent the inalso applies to the circuit of Fig.2. dication from varying during such intervals. An- Alternating currents I7, whose frequency is to be determined, are applied through the transformer 5 to the gas tube G. The gas tube ignites at a predetermined potential, limits the applied currents and produces alternating currents I9 of square wave form. If the upper electrode 1 is negative, the lower electrode II will be positive. A positive charge on the anode of D2 will permit current to flow through the diode D2 and the condenser CI will be charged with the terminal 20 positive. If on the other hand the electrode 1 is positive current flows through the diode DI and CI is charged with the terminal 20 negative. If the time constants of the circuits CI, RI, D2 and CI, DI are short compared to the highest frequency impressed on the system, a practically complete shift from negative to positive charge of CI takes place in each cycle and the average current through RI will therefore be proportional to the impressed frequency. The minimum value of C3 is determined so that the time constant of RI, be longer than the lowest frequency to be measured and the maximum value of capacity so that the build up time of the signal to be applied to the frequency measuring instrument will be that desired.

Furthermore, the charge on capacitor C3 will be negative on the terminal remote from the electrode II and positive on the terminal connected to electrode II Therefore, the polarity of C3 is such that current will flow unilaterally through the third diode D3. The current through the third diode will charge C2, which is small compared to C3. Since the capacitor C2 is shunted by the resistor R2, which may be made large, and meter I3, it follows that the time constant of the network C2, R2 may be so selected that its charge is proportional to the frequency of the pulses and its discharge or decay time, because of the isolating effect of D3 on discharge, is very slow and independent of the time constant RI, C3. Thus, the capacitor C2 is charged proportional to the beat frequency, and the meter indication will be proportional to the frequency of the applied pulses. If a few pulses are dropped out, due to signal fading or other variation, the slow discharge of the network C2, R2 (isolated as it is by the diode D3) will maintain the meter reading at or very near its initial deflection until the signals are again present, or at least for a very long period.

The time constant of the circuit RI, C3 has been considered. The circuit RI, C3 will include the capacitor 02 at least during increases in altitude. The time constant of the network RI, C3, C2 should be fast enough to follow the increases in altitude without an unreasonable delay or lag. The choice of the time constant of R2, C2 must be related not only to the signal fluctuations but also to the derivative or rate of decrease in altitude. This choice is facilitated by the fact that the fluctuations in signal strength are rapid compared to the fluctuations in altitude. It is thus possible to choose a time constant which will be short enough to follow the altitude indication reasonably well and at the same time preventing the reduction in the meter reading which would otherwise take place due to the dropping out or fading of the signals.

Thus, it will be seen that the invention is a frequency counter in which the final resistorcapacitor network is isolated from the balance of the circuit by a diode, or other unilateral rectifier, so that the network discharge or decay time is made very slow. Thus, downward variations in signal amplitude, which adversely affect a counter continuously averaging the frequency, have no deleterious effect on the counter of the instant invention.'

I claim as my invention:

1. A frequency indicating device including a current limiter responsive to the currents whose frequency is to be determined, means for deriving from said limiter currents proportional to the frequency to be determined; a meter responsive to said derived currents and having an initial deflection dependent upon the value of said derived currents, a time constant circuit associated with said meter, and means for isolating unilaterally said meter and said time constant circuit so that said initial deflection will be maintained for a substantial time notwithstanding variations in the amplitude of said derived currents.

2. A frequency indicating device including means for deriving from the currents whose frequency is to be determined a current normally having an amplitude proportional to said frequency and abnormally subject to fluctuations, a first electrical network having a time constant longer than the lowest frequency to be measured, means for applying to said first network said derived currents, a meter, a second electrical network connected to said meter and having a time constant longer than said fluctuations, and means unilaterally connecting said second electrical network and said first electrical network so that the .time decayconstant of said second network is independent of said first network.

3. A frequency indicating device including a first network having a time constant longer than the lowest frequency to be indicated, a second network having a decay time longer than the period of undesired amplitude fluctuations of the current whose frequency is to be determined, means for applying to said first network currents having normally a value proportional to the frequency to be measured, a meter effectively connected to said second network, and a rectifier connecting said second network to said first network so that the decay time of said second network is independent of the time constant of said first network.

4. A frequency indicating device including a first and a second network having respectively time constants longer than the lowest frequency to be indicated and longer than undesired variations in the amplitude of the currents whose frequency is to be indicated, means for applying to said first network a current of a value normally proportional to the frequency to be indicated, means for limiting the maximum ampli tude of said current, a meter effectively connected to said second network, and means unilaterally connecting said first and second networks so that said second network has a discharge time constant independent of the time constant of said first network whereby abnormal fluctuations below said average value have substantially no effect upon the deflection of said meter.

5. The method of indicating the frequency of a current subject to abnormal amplitude variations by means including a pair of electrical networks having dependent charging times and independent discharge times, said discharge time: being of longer duration than said abnormal amplitude variations, which includes limiting th: amplitude of the currents whose frequency 1: to be measured to obtain a current whose Vflllll is normally proportional to said frequency, applying the thus limited current to charge the first of said pair of networks, unilaterally applying the currents charging said first network to said second network, and indicating the current applied to said second network.

6. The method of indicating the frequency of a current subject to amplitude fluctuations by network to the other of said networks to charge same, and indicating frequency as a function of the current fiowing in said last named network.

7.- A frequency indicating device including a current limiter responsive to currents whose frequency is to be determined, means for deriving from said limiter currents proportional to the frequency to be determined, a meter responsive to said derived currents and having an initial deflection dependent upon the value of said derived currents, and means unilaterally isolating saidmeter so that said initial deflection will be maintained at substantially the peak value of said derived currents for a substantial time.

8. A frequency indicating device including a current limiter, means for applying to said limiter currents whose frequency is to be determined and including currents of amplitudes subject to limitation and currents of less than limitable amplitude, means including a meter for indicating said frequency as a function of the average value of the limited currents, and means for maintaining said indication at substantially its peak value during the application of said currents of less than limitable amplitude.

9. A frequency indicating device including a current limiter, means for applying to said limiter currents whose frequency is to be determined and including currents of amplitudes subject to limitation and currents of less than limitable amplitude, means for rectifying the currents derived from said limiter, means including a meterfor indicating the average value of said rectified currents, and means for maintaining said indication at substantially its peak during the application of said currents of less than limitable amplitude.

10. A frequency indication device including a first network having a time constant shorter than the period of undesired amplitude fluctuations of the current whose frequency is to be determined, a second network having a decay time longer than said period, means for applying to said first network currents having normally a value proportional to the frequency to be measured, a meter effectively connected to said second network, and a rectifier connecting said second network to said first network so that the decay time of said second network is independent of the time constant of said first network.

IRVING WOLF'F. 

